Null Values In Query

I need to create a query with the MTD field
I have created the MTD and it works!!! but now I need it to show all of the PartNum fields even if null. do I need to convert all nulls to zeros? if so how do I go about this?

Here are the fields in the query

First Field:
Field: PartNum
Table: Part
Total: Group By

Second Field:
Field: UnitPrice
Table: OrderDetail
Total: Group By

Third Field:
Field: NumOrdered
Table: OrderDetail
Total: Sum

Fourth Field:
Field: MTD: Sum([NumOrdered]*[UnitPrice])
Table: (blank)
Total: Expression

Fifth Field:
Field: CurrentYear: Year([OrderDate])
Table: (blank)
Total: Group By
Criteria: Year(Date())

Sixth Field:
Field: CurrentMonth: Month([OrderDate])
Table: (blank)
Total: Group By
Criteria: Month(Date())

When I run the query I only get 2 parts and their total. but I want all parts to show up even if they have not been ordered in the month. the parts that have not been ordered should say $0.00 in the MTD field.

Any Suggestions???